Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: III. Dynamics and Spectroscopy

In recent years, interest in carbon capture and sequestration has led to numerous investigations of the ability of ionic liquids to act as recyclable CO₂-sorbent materials. Herein, we investigate the structure and dynamics of a model physisorbing ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄C₁Im][PF₆]), from the perspective of CO₂ using two-dimensional (2D) IR spectroscopy and molecular dynamics simulations. A direct comparison of experimentally measured and calculated 2D IR line shapes confirms the validity of the simulations and spectroscopic calculations. Taken together, the simulations and experiments reveal new insights into the interactions of a CO₂ solute with the surrounding ionic liquid and how these interactions manifest in the 2D IR spectra. In particular, higher CO₂ asymmetric stretch vibrational frequencies are associated with softer, less populated solvent cages and lower frequencies are associated with tighter, more highly populated solvent cages. The CO₂ interacts most strongly with the anions, and these interactions persist for more than 1 ns. The second strongest interactions are with the imidazolium cation ring that last 100 ps, and the weakest interactions are with the cation butyl tail that persist for 10 ps. The principal contributors to spectral diffusion of the CO₂ asymmetric stretch vibrational frequency due to the dynamical evolution of the solvent are through Lennard-Jones interactions at short times and electrostatics at long times.